Rechargeable electrochemical storage systems are presently becoming increasingly important in many areas of everyday life. In addition to the long-standing applications as automobile starter batteries and as an energy source for portable electronic devices, considerable growth is predicted in the future for electric automobile drives and for stationary energy storage. Traditional lead/sulfuric acid accumulators are not suitable for the new applications because their capacity is far too low, and they cannot be cycled frequently enough. In contrast, the best prospects are seen with lithium batteries.
However, lithium accumulators according to the prior art likewise have too little energy storage capacity for many applications. Present lithium-ion batteries have specific energy densities between approximately 100 and 250 Wh/kg. In addition, they usually contain costly elements such as cobalt and/or nickel. Lithium/sulfur and lithium/air systems have much higher (theoretical) energy densities:
Theoretical energy densityBattery systemWh/LWh/kgLi ion (LiC6/Ni, Mn, Co oxide)1710510Lithium/sulfur27102450Lithium/air5830
The technical challenges in the development of Li/air systems are still so great that a marketable system is not expected for at least another 10-20 years (M. Jacoby, Chem. Eng. News Nov. 22 (2010) 29-31). The prospects for the lithium/sulfur system appear to be much more favorable. One drawback of this system is the use of lithium metal anodes. Lithium metal is relatively costly compared to saline materials or the graphite used in lithium-ion batteries. In addition, this battery has the further disadvantage that it loses capacity too rapidly during charging and discharging.
It has therefore been proposed to assemble the lithium/sulfur battery in the discharged state; i.e., a lithium-free (or low-lithium) material such as a tin/carbon composite is used as the anode, and lithium sulfide is used as the cathode (B. Scrosati, Angew. Chem. 2010, 122, 2421-2424). Unfortunately, this battery configuration has likewise proven to have insufficient cyclical stability. The main reason is that soluble oligosulfur compounds (Li2S3 and Li2S4, for example) may form during cycling. As a result, the cathode loses redox-active material (Y. Li, J. Power Sources 195 (2010) 2945-2949; D. Aurbach, J. Electrochem. Soc. 156 (8), A694-A702 (2009)). To improve the conductivity of the cathode material (sulfur or lithium sulfide), formation of a composite with carbon is often resorted to. Thus, T. Takeuchi has reported that commercially available sulfide powder may be coated with graphitic carbon in an arc plasma process (J. Electrochem. Soc. 157 (11) A1196-A1201 (2010)). However, such coating processes are energy-intensive and require expensive coating equipment and high-vacuum technology, and consequently entail high costs. Another approach to producing Li2S/C composites is to grind commercially available lithium sulfide powder in a ball mill, for example (B. Scrosati, Angew. Chem. 2010, 122, 2421-2424). Grinding processes likewise have a fairly complicated technology and require the availability of finished components. Lithium sulfide is obtainable from the chemical trade, but only at high cost, for example 560.00 for 50 g from Alfa Aesar (list price, 2011-2013 catalog edition). Lastly, it is known that the reaction of lithium with sulfur in boiling naphthalene results in a main product having the approximate composition of Li2S and containing impurities of free metal (elemental lithium), carbides, and polysulfides (T. G. Pearson and P. L. Robinson, J. Chem. Soc. 1931, 413-420).